BOLT 2: specify nSequence explicitly.

We don't care, as long as it's RBF-able.  This will be nicer for
Taproot when mutual closes are otherwise indistinguishable from normal
spends.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

02-peer-protocol.md

@@ -810,6 +810,7 @@ This process will be repeated every time a `shutdown` message is received, which
 2. data:
    * [`channel_id`:`channel_id`]
    * [`u64`:`fee_satoshis`]
+   * [`u32`:`sequence`]
    * [`closing_tlvs`:`tlvs`]
 
 1. `tlv_stream`: `closing_tlvs`
@@ -842,6 +843,7 @@ Both nodes:
 The sender of `closing_complete` (aka. "the closer"):
   - MUST set `fee_satoshis` to a fee less than or equal to its outstanding balance, rounded down to whole satoshis.
   - MUST set `fee_satoshis` so that at least one output is not dust.
+  - MUST set `sequence` to a value other than 0xFFFFFFFF.
   - MUST use the last send and received `shutdown` `scriptpubkey` to generate the closing transaction specified in [BOLT #3](03-transactions.md#closing-transaction).
   - If it sets `signature` fields, MUST set them as valid signature using its `funding_pubkey` of:
     - `closer_no_closee`: closing transaction with only the local ("closer") output.
@@ -863,6 +865,8 @@ The sender of `closing_complete` (aka. "the closer"):
 The receiver of `closing_complete` (aka. "the closee"):
   - If `fee_satoshis` is greater than the closer's outstanding balance:
     - MUST either send a `warning` and close the connection, or send an `error` and fail the channel.
+  - If `sequence` is equal to 0xFFFFFFFF:
+    - MUST either send a `warning` and close the connection, or send an `error` and fail the channel.
   - Select a signature for validation:
     - if the local output amount is dust:
       - MUST use `closer_no_closee`.

03-transactions.md

@@ -394,15 +394,15 @@ than twice `dust_limit_satoshis`.
 
 This variant is used for `closing_complete` and `closing_sig` messages (i.e. where `option_simple_close` is negotiated).
 
-In this case, the node sending `closing_complete` ("the closer") pays the fees, and the sequence is set to 0xFFFFFFFD to allow RBF.  The outputs are ordered as detailed in [Transaction Output Ordering](#transaction-output-ordering).
+In this case, the node sending `closing_complete` ("the closer") pays the fees, and the sequence specified to allow RBF.  The outputs are ordered as detailed in [Transaction Output Ordering](#transaction-output-ordering).
 
 The side with lesser funds can opt to omit their own output.
 
 * version: 2
 * locktime: 0
 * txin count: 1
    * `txin[0]` outpoint: `txid` and `output_index` from `funding_created` message
-   * `txin[0]` sequence: 0xFFFFFFFD
+   * `txin[0]` sequence: `sequence` from `closing_complete` message
    * `txin[0]` script bytes: 0
    * `txin[0]` witness: `0 <signature_for_pubkey1> <signature_for_pubkey2>`